To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). Modern WDM systems have a high traffic capacity, for example, a capacity to carry 64 channels at 10 gigabits per second (hereinafter Gb/s).
The optical fiber transmission system may include a relatively long trunk segment that may be terminated at a transmitting and/or receiving trunk terminal. The optical fiber transmission system may further include one or more branching units situated along its trunk. Each branching unit (BU) may be connected to a branch segment that terminates in a transmitting and/or receiving branch terminal. Each BU may include one or more optical add/drop multiplexers (OADM). Channels may be added to and/or dropped from the trunk segment of the optical transmission system via the OADMs. Accordingly, the system may be dynamically loaded and unloaded with signal channels as they are added and/or dropped at the BUs.
When the information channels are transmitted over long distances or between links of optical fiber cable, one or more amplifiers may be provided to compensate for signal attenuation. The amplifiers used in some WDM systems cannot easily be modified, and may be initially configured to support a fully loaded link (e.g., 64 channels, each channel carrying 10 Gb/s). In general, it may be desirable that the power per channel be sufficient to provide an adequate signal-to-noise ratio in the presence of the amplified spontaneous emission (ASE) noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers may thus be configured to provide an optical output signal at a nominal total optical power.
The nominal amplifier output power level may be insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the total amplifier output power may change very little around the nominal output power level. As additional channels are added, e.g. at a branching unit, the optical output power per channel may decrease. As channels are dropped, the optical output power per channel may increase.
In a fiber optical communication network the fiber medium is non-linear. At high optical powers (e.g., more than 10 mW per channel), the optical signal may experience more distortion than at low optical powers (e.g., less than 1.0 mW per channel) which results in transmission penalty. Therefore when channels are dropped the value of optical channel power may increase, and network communication performance may suffer. Partial channel loading of a chain of optical amplifiers may result in undesirable noise accumulation and gain reshaping effects that also degrade channel performance